Simultaneous dual seismic spread configuration for determining data processing of extensive seismic data

ABSTRACT

Signals from the same seismic source are simultaneously recorded from two groups of seismic detector arrays. The two groups have detector arrays and array intervals unique to each group. The group closest to the seismic source accentuates shallow reflections and is characterized by: (1) closely spaced detectors in each array forming short arrays with short distances between array centers, (2) nearness to the source, (3) usually sampled and filtered to resolve high-frequency data, and (4) a low order of multiple coverage. The group more remote from the seismic source enhances the deeper reflections and is characterized by: (1) long arrays with long distances between array centers, (2) a location a mile or two from the source, (3) sampling and filtering to resolve low-frequency data, and (4) a high order of multiple coverage. A factor, i.e., number of detector arrays times the spacing between array centers divided by the multiplicity of coverage, for one group must be equal to the corresponding factor for the other group, although the multiplicities of coverage of the two groups are very different. The near group data determines the processing techniques and corrections for the mass of data from the far group.

United States Patent [72] Inventor Roy G. Quay San Antonio, Tex.

[21] Appl. No. 888,044 [22] Filed Dec. 24, 1969 [45] Patented Oct. 12,1971 [73] Assignee Petty Geophysical Engineering Company San Antonio,Tex.

[54] SIMULTANEOUS DUAL SEISMIC SPREAD CONFIGURATION FOR DETERMINING DATAPROCESSING OF EXTENSIVE SEISMIC DATA Primary ExaminerRodney D. Bennett,Jr. Assistant ExaminerN. Moskowitz AttorneyWatson, Cole, Grindle &Watson ABSTRACT: Signals from the same seismic source are simultaneouslyrecorded from two groups of seismic detector arrays. The two groups havedetector arrays and array intervals unique to each group. The groupclosest to the seismic source accentuates shallow reflections and ischaracterized by: (l) closely spaced detectors in each array formingshort arrays with short distances between array centers, (2) neamess tothe source, (3) usually sampled and filtered to resolve highfrequencydata, and (4) a low order of multiple coverage. The

13 Claims, 6 Drawing Figs.

group more remote from the seismic source enhances the [52] U.S-CI340/7, deeper l-eflections and i characterized 1 long arrays 340/155 MCwith long distances between array centers, (2) a location a [51] IliLClG01v 2/16 mile or two from the Source, 3 sampling and filtering to [50]Field of Search 340/ 15.5 resolve 10w frequency data and 4 a high orderof multiple 7 coverage. A factor, i.e., number of detector arrays timesthe spacing between array centers divided by the multiplicity of [56]References Cited coverage, for one group must be equal to thecorresponding UNITED STATES PATENTS factor for the other group, althoughthe multiplicities of 2,757,356 6/ l956 Haggerty 340/7 coverage of thetwo groups are very different. The near group 2,759,551 8/1956 Carlisleet al. l8 l/.5 data determines the processing techniques and correctionsfor 3,096,346 7/1963 Savit et al. 340/ 15.5 the mass of data from thefar group.

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SHEET 3 CF 3 V SOURCE LOCA'HON O DETECTOR. LOCA'HON AA 5\ 51 53 54 BS 56o o o 0 COMMON REFLECTION PO\NT an 51 bb bbsw .bb 5. .ZZZ: 1 4 .ddso5... 1. .2 21 99 5\ 99 5'! a .99 E 51 ga, iis\ ii s1 u ()5 1 l 53.51 W.it: Q .ma .L mp1s) -mm69 mm SI -nnl0 SAMPLE AND RECOYZD SAMPLE ANDRECORD AT SHORT ME AT LONG 'HME \NTERVALS \NTERVALS U ms) (4 ms WGHFREQUENCY WW R QUENCY BAND PASS FHIERS BAND PASS FHIERS u 10 camerasorsuom CENTERS or- LONG DETECTOR ARRAYS DETECTOR. ARRAYS M/l/E/VTOB,076. 0a, 8) (/6 11/ w wflm 7 rrazA/frs SIMULTANEOUS DUAL SEISMIC SPREADCONFIGURATION FOR DETERMINING DATA PROCESSING OF EXTENSIVE SEISMIC DATAThis invention relates to seismic surveying and, more specifically, to amethod and apparatus for providing dual series of seismic detectorarrays. The invention disclosed herein is primarily useful in marineseismic exploration; however, those skilled in the art will recognizethat it can be employed with suitable modification for seismicexploration on land. In marine seismic exploration it is customary touse a cable which contains pressure-activated devices for detectingseismic signals. It is likewise customary to have such a cable deployedbehind a seagoing vessel for the purpose of detecting the seismicsignals generated on or near the vessel by any number of known sourcesor devices. Such pressure-activated detectors are arranged in an array,or pattern, as, for example, described by Parr in US. Pat. No. 2,698,927or Kearns in US. Pat. No. 3,335,401. The output of a group of individualdetectors in an array is then recorded on a given channel. A series of24 or more arrays are usually uniformly spaced along the cable.

In accordance with the present invention, a marine cable simultaneouslyemploys two groups of detector arrays, each of these groups has detectorarrays and intervals between array centers difi'erent from the otherarray. The longer group of detector arrays is the type commonly used formultiple coverage. It must be long enough to attenuate undesiredmultiple reflections. Frequently, this group extends for 2 miles fromthe source and consists of 24 detector arrays with a 440 -foot spacingbetween array centers. The detectors in each array are distributed overmost of this 440 -foot interval to attenuate low-frequency (longwavelength) noise transmitted along the the spread. A shorter group ofdetector arrays may consist of six detector arrays with about 73 feetbetween array centers. The detectors in each array are distributed overmost of this 73 -foot interval. The total length of this array group inthe above example is less than 440 feet. A longer array is not requiredfor this short group because the low frequencies may be removed byelectrical or digital filters or other means described herein. The440-foot array, used for the longer group, would be too long to detectthe high-frequency component of the shallow reflections; the signalsfrom the source to shallow reflecting horizons and back to the variousdetectors in this long group would be out of phase and severelyattenuated when combined within a given detector array.

The shorter group is placed as close as practical to the seismic source.In this manner, the normal moveout (NMO) is very small and even if thevelocity of seismic propagation to the reflecting horizon is notproperly known, any error in velocity would introduce only negligibledifferences in NMO. The use of a higher frequency pass band for theshort spread also permits placing the shorter group closer to theseismic source and a shorter array than would be possible of alowfrequency pass band were used. Seismic recordings containing highfrequencies are desirable since they provide greater resolution of theshallow reflections than would be possible with low frequencies. Greaterresolution is particularly desirable in marine exploration since thestrata immediately below the water bottom consists of thin layers andthe velocity, low in nature varies quite rapidly from place to place dueto various geological phenomena. For example, there are numerous ancientriver channels filled with silt and other debris which cause rapidchanges in sub-bottom formations. A further advantage of the shortergroup is that the reflection points recorded therein will be closertogether than if the longer spread were used. This permits mapping thelow-frequency in sub-bottom conditions in greater detail and reduces thepossibility of ambiguity because of rapid changes in reflection time.Such ambiguity does not allow the reflections to be correlated from onechannel to another. The shorter group radically contrasts with thelonger group which is from a mile to 2 miles in length and which detectsthe low-frequency components of the seismic signals. The well-knownhorizontal stacking techniques are required for the longer group toattenuate multiple reflections from shallow reflecting horizons.Multiple reflections overlap the desired primary reflections from deeperreflecting horizons. in general, twelvefold to twentyfourfold coverageis used in most marine seismic exploration. This means for every commonreflection point on the composited section there are at least 12 to 24separately recorded channels which have been combined. The processing ofthis data from the longer group requires some type of correction guideto expedite the processing of the data in a digital computer. Thelimited amount of data from the shorter group is used to detennine thestatic correction due to the weather layer below the bottom and alsoserves as a guide to the complex processing of the mass of data from thelonger group.

The present invention provides the capability of simultaneously yieldingtwo different folds of seismic coverage, e.g. single coverage (onefold)with a larger fold, such as twenty-fourfold coverage. By a systematicarrangement of the source interval, the single coverage may become ahigher fold (two, three, etc.).

It is a primary object of this invention to provide for the simultaneousrecording of difi'erent folds of seismic coverage wherein onefoldcoverage is obtained from a group of detector arrays which enhancessignals from shallow reflections and the other fold coverage isdetermined from a group of detector arrays which enhances deepreflections.

It is a further object of this invention to provide an improved methodand apparatus for enhancing deep reflections obtained from a marineseismic survey.

The foregoing objects and advantages will be more clearly understoodfrom the following description when taken in conjunction with thedrawings wherein:

FIG. la illustrates a seismic cable having two groups of detector arraysarranged according to this invention and also represents the subsurfacecoverage for any given source location;

FIG. 1b illustrates the varying degrees of normal moveout obtained fromone source location with segments of both groups of detector arrays asused in FIG. 12;

FIG. 2 illustrates the sequence of successive source and detector arraylocations for the method and apparatus of the invention;

FIG. 3 is illustrative of the subsurface coverage of successive sourcelocations using the dual group configuration in accordance with theinvention wherein the source locations are systematically spaced so asto simultaneously yield a singlefold coverage plus a multiplefoldcoverage;

FIG. 4 illustrates an alternative configuration for the marine dualgroup of detector arrays in accordance with the configuration of FIG. 3;and

FIG. 5 illustrates the different filters and recoding systems for theshorter and longer groups of detector arrays.

In the following description, 24 stations are used to illustrate theprinciples of the invention. However, it is to be understood that theinvention may be applied to any given number of detector arrays.

With respect to FIG. 1a boat I01 tows cable 102 at a selected depthbelow the water surface 103. Centers 11-22 of geophone array shown alongcable 102 and other centers 23-24 are on the cable to the right of theillustrated cable segment. The spacing between array centers 17-34 islarge, such as 440 feet, so that undesired multiple reflections areattenuated by using the horizontal stacking techniques known to theseismic art. An appreciable number of seismic detectors, such as 30 aredistributed along the cable symmetrically from the array center I7 andextend nearly half of the distance to the next array center 18. Similararrays of seismic detectors are distributed on the cable 102 about arraycenters 18-34. This group of seismic detector arrays is hereinaftercalled the longer group. A seismic source 104 radiates a seismic impulseinto the Water and signals received by the longer group 17-34 aretransmitted along conductors in cable 102 to recording equipment in boat101. The raypaths are shown for seismic energy from source 104 down toreflection points 17 and 22' on reflecting formation 105 and then up toarrays 17 and 22. These paths are through the sea down to the sea bottom106 and then through various layers below the sea bottom. These raypathsare illustrated for only one of many reflecting formatrons.

A shorter group of seismic detector arrays are distributed about arraycenters 11-16. This shorter group may be on the same cable 102 as thelonger group or on a separate nearby cable, not illustrated. Reflectionraypaths are shown from the source 104 down to reflection points 11' and16' and back up to arrays at 11 and 16. The spacing of these arrays mustby proper for the number of arrays and the multiplicity of coverage, asdiscussed hereinafter.

The spacing between detector arrays and the number of arrays in a groupis determined by the equation:

1 N, D,/M,=N D /M Subscript 1 denotes the shorter group and subscript 2denotes the other series; N is the number of detector arrays in thegroup; D is the distance between centers of arrays; and M is themultiplicity of coverage obtained. In this case, if there are l8 arraysin the longer group with 440 feet between centers, the shootingprocedure is to obtain eighteenfold coverage from the longer group. Ifthere are six arrays in the shorter group and singlefold coverage isdesired from that group, the spacing D is determined from equation l)as:

6XD,/l=l 8x440 feet /1 8 or D, is approximately equal to 73.3 feet.

The spacing of the detectors in an individual array is determined bystandard operating techniques based on the frequency range of the passband of the recording system. The shorter group uses high frequencies sothe detectors in an array are close together and extend over a limiteddistance to record the shallow high-frequency reflections withoutappreciable attenuation. The longer group has a lower frequency responseso the detectors are farther apart, but they must be distributed overseveral hundred feet in order to attenuate the lowfrequency (longwavelength) noise.

In order to properly record the high frequencies in the shorter group,it is preferred to have the high-cut filter at a higher cutoff frequencythan normal, such as 300 Hz., and to use a l -millisecond sample ratefor the seismic signals from each array. For the deep reflections fromthe longer series, it has been common to use a 75 Hz. high-cut aliasfilter and a 4- millisecond sample rate for the seismic signals fromeach array.

As has been discussed previously, the shorter group provides a betterresolution of shallow data and accommodates the high-frequencycomponents of the various reflections. Thus, the static corrections areaccurately determined to enhance the output information obtained for thelonger group for obtaining multiple coverage. Furthermore, thesignificant increase in resolution of the shallow data obtained by theshorter spread provides an optimum representation of the geologicformations adjacent to the sea bottom.

Within the present invention, there are several advantages which will beevident to those skilled in the art. First, my invention will enable theseismologist to examine the shallow reflecting horizons through moreclosely spaced reflections points while also recording the deeperreflections, through the longer interval group, for the compositing ofreflections in the stacking process. Furthermore, the higher frequenciesreflected by the shallow reflecting horizons will be recorded forobservation in the form of singlefold or other low multiplicities ofcoverage. Finally, the seismologist, by a utilization of the presentinvention, can obtain a substantial reduction in seismic data processingcorrective techniques by relaying upon information obtained from hisstudy of the common reflection points of the low multiplicity coverageas a guide to the best method and mode of processing the highmultiplicity coverage recorded from the longer group.

In order to attenuate second and higher order reflections, commonlyreferred to as multiple reflections, and also to accentuate primaryreflections, the length of the longer group must be at least 1 mile inlength and it may extend lk or two miles in length. Thus, in a 24-arraygroup, each detector array interval would range approximately from 220feet to 330 feet to 440 feet, respectively.

In areas where the geologic formations are inclined at high angles,i.e., the subsurface layers are inclined at an appreciable angle withrespect to the horizontal surface, as shown by Mayne, Oil & Gas Journal,Sept. 29, 1969, the distance between reflection points must be greatlydecreased to permit resolution of steeply dipping reflecting horizons.Thus, the corrected time from one reflection point to the next oradjacent reflection point may be one-half the time involved to record agiven signal frequency, i.e., the corrected time from one reflectionpoint to the next reflection point may be onehalf of one cycle of thegiven signal. This correction time might also be more than one-halfcycle of the particular signal.

To those skilled in the art, the fact that the subsurface layers havevarying velocities is obvious. Furthermore, it is a wellestablished factthat the lower velocity layers are adjacent to the sea bottom.Therefore, if large variations in the low velocity zone exist overrelatively short lateral distances along the traverse, correlation ofthe common reflection points may be ambiguous and totally misleading.This ambiguity would exist in the shallow reflecting horizons and wouldresult in a given reflection being shifted more than one-half of onecycle between two adjacent reflection points. A reduction in thedistance between adjacent reflection points can resolve this ambiguity.

To achieve this reduction in horizontal distance between two givenreflection points, there are several possible approaches: first, by acloser spacing of the source locations, i.e., a reduction in thehorizontal distance between adjacent reflection points can beaccomplished; furthermore, a reduction in the detector locations, i.e.,a reduction in the detector interval, or, as commonly referred to, thespread interval; finally, a reduction in the distance between adjacentreflection points can be resolved by combining the heretofore mentionedtechniques. For example, a decrease in the source interval and adecrease in the detector interval can result in a reduction in thedistance between adjacent reflection points. Thus, the need for a dualgroup configuration is realized and can be accomplished by theembodiment herein and the exploitation of the present invention.

In actual practice, the array of detectors which are sued as a group forrecording the higher frequency components of the shallow reflectionscannot physically be spread over as long a distance as that which isrequired to attenuate noise in the low frequencies of the deeperreflections. The higher frequency components of the shallow reflectionscan readily be recorded, without great difiiculty, on a few channels,e.g., detector arrays 11 through 16 of FIG. 1a. It is noteworthy that inutilizing the above-mentioned method of recording, a large number ofrecordings, i.e., a great mass of data, need not be recorded. Normally,the seismic recording channels are sampled only at the time limit whichborders on deterioration of the highest frequencies of the deeperreflections. Thus, a sample rate of 4 milliseconds is used, i.e., eachof the seismic recording channel is sampled ever 4 milliseconds.However, in some circumstances, a given set of conditions may demand a lmillisecond or a 2-millisecond sample rate. In order to record thehigher frequency components of the shallow reflections, it is necessaryto sample each of the seismic recording channels at a high rate, i.e.,preferably at every 1 millisecond. It is wellknown that lessdifferential moveout can be tolerated for st stacking higherfrequencies; however, more differential moveout is actually encounteredfrom shallow reflections for the same source-to-detector distance. Thisfact is well illustrated by FIG. 1b. Moreover, high-frequencyreflections are usually better from a nearly vertical path and are verypoor at wide angles. Such an arrangement is illustrated in FIG. 10,wherein the paths to arrays 11-16 are more desirous for thehigh-frequency shallow reflections.

- Because reflections received by the shorter group follow near verticalraypaths, discrepancies in velocity do not appreciably alter the truereflection times. Thus, the need for a high degree of accuracy invelocity when processing the common reflection point traces is notacute. In reality, if the shorter group approaches the idealconfiguration for the given set of circumstances, no time variablecorrections will be necessary. Upon the inspection of the seismic datarecorded by the shorter group, the data will illustrate any steeplyJdipping beds; any abnormal weathering layers, i.e., areas of largedifferential velocities; any severe reverberation; and any other noise.Thus, the seismic data recorded by the longer group can be subjected tothe appropriate guidelines as determined by the shorter group. Theseismologist, by examination .of the seismic data resulting from theshorter group and from the longer group, can compare the two sets ofdata and determine the appropriate corrections for the seismic datarecorded by the longer group.

The deeper reflections are predominantly of a low frequency andtherefore require that the elements in a detector array be spread overgreater distances in an effort to attenuate the horizontally propagatedlow-frequency energy that results in the distortion and destruction ofmeaningful data from a subsurface common reflection point. Frequencyfiltering generally cannot be used to attenuate such undesiredlowfrequency noise since the frequency of the noise lies within therange of useful reflections received from the deeper fonnations and,consequently, it is necessary to have the longer array to attenuate thelong wavelength, low-frequency noise, and to accentuate deeperreflections.

Multiple reflections are known to be severe when the lowfrequency deeperreflections are received, and a combination of reflections fromappreciably different angles is the preferred procedure to attenuate thelow-frequency components of the reflections and to accentuate andenhance the valid deeper primary reflections. The time differentialbetween primary reflections and multiple reflections must be comparableto, or greater than, the longest period in the multiple reflections toadequately attenuate the multiples. Similarly, when the velocitygradient is small, the spread differential must be great to attenuateunwanted signals. Therefore, the longer group represented by detectorarray stations 17-34 is necessary. 1 If the overall spread length of thedual series configuration is not sufficient for multiple suppression,extension sections may be inserted between the shorter and longergroups, i.e., between detector array stations 16 and 17 as illustratedin FIG. la. Such extension sections would not contain any detectors.Thus, the distance between the shorter and longer group detectorconfigurations may be varied to suit a given set of conditions or toaccommodate the specific needs of the seismologist.

In recording the seismic infonnation, the shorter group, for limitedcoverage, may be recorded on another set of instruments so thatinstrumentation and techniques applicable to the peculiarities andcharacteristics of each group may be utilized, for example, differentfilters and response equipment for each group. Furthermore, there may betwo cables and two recorders, or one cable and one recorder. Likewise,any additional combination worthy of a given set of circumstances may beutilized as desired.

FIG. 2 illustrates successive source locations and group configurations.The pattern illustrated for the longer group is in accordance with thatof W. Harry Mayne as set forth in the Dec. 1962, issue of Geophysics,Volume 27, No. 6, at pages 927-938. FIG. 2a illustrates the position ofthe source at A, the closer group consisting of detector arrays l l-16and a portion of the longer group represented by detector arrays 17 and18. In offshore use, the seismic source generally generates a seismicsignal as the cable assembly is being pulled through the water. However,to simplify the present discussion, the Figure represents the conditionsas if the cable were motionless in the water when the source is excited.When the cable has been advanced the proper distance along the traverse,the source is excited at source location B as illustrated in FIG. 2b. Inthis particular case, detector array station 13 is at the former sourcelocation A. A longer interval could be selected between the source anddetector array 11; the interval illustrated is merely convenient for theillustration. Successive source locations along the traverse areillustrated in FIGS. 20 through 2h by source locations C through B,respectively.

FIG. 3 illustrates the location of reflection points obtained from aseries of seismic sources along a traverse for the source and detectorarray locations in FIGS. 10 and 2. FIG. 3a is similar to FIG. la, but isdrawn at the same horizontal scale as the remainder of FIG. 3. Thesignals received by the arrays from source A are recorded. In FIG. 3b,the reflection points are along the same traverse but placed lower inthe Figure for clarity. The seismic source and detector arrays have beenmoved left so that the source is at B. The distance between A and B isone-half of the distance between consecutive detector array centers 17and 18 in the longer group. This distance places the reflection pointB-18 from source B to detector array 18 at the same location along thetraverse as reflection point A-17 from source A to array 17. Similarly,reflection point B-19 is at the same point as reflection point A-18 andso on to B-34 at the same point as A-33 (8-34 and A-33 are not shown inthe Figures). The signals received by the detector arrays from source Bare recorded. The source and detector arrays are moved, shifting thereflection points as illustrated in FIG. 3c, and recording made fromsource C. The process is repeated down the traverse as illustrated inFIGS. 3d-3u. Reflection points A-l7, B-18, C-l9,...,R-34 are at the samepoint hence common reflection point (CRP) and is indicated as CRP-l.Other CRPs are shown such as CRP-3 consisting of C-17-T-34. The datafrom each CRP is corrected for path length and then combined by theprocess of horizontal stacking. The source and all of the detectorarrays are moved along the traverse a distance equal to one-half of thedistance between detector array centers in the long group, the sourceexcited and the data received by the detectors is recorded, and theprocess is repeated along the traverse. This process results in amultiplicity of coverage in the longer group which is equal to thenumber of detector arrays in that group. In this case, eighteenfoldcoverage in the long group is obtained from the l8-detector arrays inthe longer group While this has been illustrated with cable and sourcesat rest during the recording, both could be in continuous movementthrough the process. The reflection points resulting from deeperreflections would be shifted laterally in time along the traverse, butonly to a small degree.

In FIG. 3a, seismic energy from source A is reflected at reflectionpoint A-16 to detector array 16. Similar reflection points exist fordetector arrays 11-15. The path for seismic energy from source A toreflecting point A-ll and to array 1 1 is illustrated. Reflection pointsA-16-A-1l are uniformly spaced along the profile since the detectorarrays 11-16 are uniformly spaced. The source is moved to B as shown inFIG. 3b and reflection points B-16-B-1 l are obtained. If the spacingbetween array centers is selected as previously described, the distancefrom reflection point A-ll to reflection point B-16 will be the same asthe distance between other consecutive reflection points, such as A-l2to A-II.

Detector arrays 11-16 are used to obtain singlefold coveragesimultaneously with the high multiplicity of coverage from detectorarrays 17-34.

The mass of data involved is tremendous; each individual sample maycontain l8 binary bits in floating point form; there may be fifteenhundred to three thousand samples per reflection point; and in theillustration, there are 18 reflection paths per common reflection point,with another CRP every 220 feet along the traverse; and at reasonableboat speeds this distance is covered in 22 seconds. This could be aboutI million bits of binary, floating point, raw data collected every 22seconds. The work done in processing this mass of raw data is fantastic,for example, an autocorrelation of just one sample includes threethousand times three thousand or 9 million multiplications, additions,and then these samples must be stored.

According to my invention, the processing accuracy can be greatlyimproved with a fantastic reduction of time and equipment. This includesthe recording of the data from a shorter group of seismic detectorarrays simultaneously with the recording of the data from the longergroup of seismic detector arrays. As previously discussed, the number ofarrays, the distance between array centers and multiplicity of coverageon the shorter group must be appropriate for the same factor in thelonger group. Other features between the groups are different aspreviously described. The spacing in the shorter group for the arraycenters 11-16 is shown in FIG. 3a. The total distance from array center11 to array center 16 in the shorter group is less than the distancebetween consecutive array centers, such as array centers 17 to 18, inthe longer group. This provides a series of reflection points A-11-A-16.For the illustrated arrangement reflection point A-16 is at CRP-3,although by shifting the entire shorter group along the cable anydesired reflection point on the shorter group is made to coincide withthe CR? of the long spread. Single coverage on the shorter group isillustrated in FIGS. 1, 2 and 3, that is, each reflection point in theshorter group is recorded only once. These reflection points areuniformly spaced along the traverse such as from reflection point A-llto A-l2 and from 8-16 to 8-1 1. Therefore, six detector arrays for theshorter group are used, but data from six times as many reflectionpoints, as recorded from the longer groups has been obtained.Furthermore, the shallow data from the shorter group is superior to theshallow data from the longer group, because better shallow reflectionsare obtained from near vertical raypaths. The high frequencies requiredfor good resolution can be obtained by frequently sampling the data;hence it is very desirable to process only selected data, not the entiremass of data. Small moveout (NMO) on the shorter group permits excellentNMO corrections due to little or no signal distortion. Variations intime are due to dipping formations or weathering on the seismicrecordings which have had NMO removed. The continuous repetition of theshorter group along the traverse permits determination of weathering byremoval of the dip. The short arrays do not smear or average thesubsurface nor does it attenuate high-frequency reflectrons.

FIG. 4 illustrates twofold coverage resulting from the shorter group,while the longer group affords an eighteenfold coverage. By utilizingequation (I), the spacing between detector arrays and the number ofarrays can be determined. In the cited example, FIG. 4, if there are 18detector arrays in the longer group with 440 feet between array centers,the shooting procedure will result in eighteenfold coverage from thelonger group Thus, if there are six detector arrays in the shorter groupand twofold coverage results from this group, equation (1) yields thefollowing:

6 D,/2=l8X440 feet/l8 or D is approximately equal to 146.7 feet. Twicethe detector array spacing for the shorter group of FIGS. 1a, 2, and/or3 is required in FIG. 4 to result in twofold coverage, all other factorsremaining constant.

In a given set of circumstances, the detector array length for theshorter group remains constant, and the number of detector arrays isincreased. Thus, the results are uniform with the heretofore mentionedFIG. 4. By algebraic expression:

Hence, twofold coverage in the shorter group is achieved by alterationof either the number of detector arrays or alteration of the distancebetween detector array centers or alteration of both.

Thus, if a multiplicity of coverage higher than singlefold coverage isdesirous in the shorter group, it can easily be achieved. If, however,at a later date, the seismologist desires only singlefold coverage and ahigher multiplicity of coverage has been recorded, the appropriatechannels may be deleted in the data processing of the seismic channels.Such a process is much more feasible than reexploring the same traverse.

The fact is emphasized that in the heretofore mentioned formula, N doesnot refer to those detector arrays which may or may not be used ascommon detector arrays to two or more group configurations. I.e., thesedetector arrays are not tie" detector array groups. Likewise, it isnoteworthy that N does not refer to any overlapping of detector groupsfrom one group configuration to any subsequent group configuration.

FIG. 5 illustrates, in block diagram format, suitable recordingapparatus wherein the centers of the long detector arrays are indicatedat 17-20. Each of these detector arrays may consist of 30 or moredetectors distributed over a distance of a few hundred feet. The signalsfrom all of the detectors in an array are combined and transmitted byconductors to the recording station, on a boat in the case of a marineoperation. The signals from each detector array such as 17 are filteredthrough an appropriate filter selected in accordance with the propertiesof the area and the depth of the zone of interest. If the zone ofinterest is typical of present seismic exploration for oil (maximumrecord time about 6 seconds), the filter pass band may be in the orderof 5 to 75 Hz. and in this case a sample of each array every 4milliseconds would suffice. If the zone of interest was shallower, ahigher frequency pass band would generally be preferred. The centers ofthe short detector arrays are indicated at 11-16. Each of these detectorar rays may consist of 20 detectors distributed over a distance of 40 tofeet. The signals from all of the detectors in an array are combined andtransmitted to the recording station on the boat. The signals from eachdetector array such as 11 are filtered through an appropriate filterselected in accordance with the depth of the water, properties of thearea, and purpose of the survey. Again, in the seismic exploration foroil, this filter might be 40 to 300 Hz. and the signal from eachdetector array would be sampled every I millisecond.

If the purpose of the survey is to locate shallow minerals or otherobjects at shallow depths, both arrays are shortened and higherfrequencies used. If the target is very deep, the arrays and lowerfrequencies are used.

There are several points that should be noted in the illustrateddisclosure of FIGS. 1-3. The combined overall length of all of thedetector arrays in the shorter group is less than the center-to-centerdistance of two consecutive arrays in the longer group. In this case, itmeans that six times as many reflection points are obtained from theshorter group as from the longer group of detector arrays. This makes itpossible to map the sediments immediately below the bottom at six timesas many locations as with the longer group, therefore in much greaterdetail and avoiding the ambiguity due to the rapid changes in depth orweathering.

The second point is that with a single series of shots, it is possibleto simultaneously record both a low multiplicity of coverage for theshallow reflections and a high multiplicity of coverage for the deeperreflection. This is a requirement for the removal of undesired multiplereflections from the desired deep primary reflections by the process ofhorizontal stacking.

A further advantage of this invention is that data from the shortergroup of detector arrays may have an approximate NMO correction appliedand then used as true time. This is possible since all of the detectorsin the shorter group are near the seismic source, hence NMO correctionsare very small and generally NMO errors are negligible. This is notpossible for data from the longer group of detector arrays, since thereare very large values of NMO and hence large errors in NMO due to theassumption of a vertical velocity. Static corrections computed from dataobtained with the shorter group are applied to the longer group ofdetector arrays. Then the NMO and average velocities are determinedautomatically for the seismic data derived from the longer group.

A further use of the shorter group of detector arrays is that the outputof either a single array or a combination of a few arrays (after NMO isapplied) is used to predict the type of processing needed for the longergroup of detector arrays. One of the most effective methods is to takean autocorrelation of the data from the shorter group and then determinethe uniform repetition time of the reflections, typical of multiplereflections. If the multiple reflections are close together as indicatedby only small uniform intervals between the peaks of the autocorrelationcurve, then the length of the deconvolution operator has a short timeduration. If, however, there are only long time intervals between therepetitive peaks of the autocorrelation curve, such as one-half asecond, then it is necessary for the deconvolution operator to have along time interval, such as a little more than half a second. Ifrepetitions have both short and long time intervals, then thedeconvolution operator would require both closely spaced values and anextension over a long time interval. Thus, analysis of a limited amountof data determines the length and number of points in the deconvolutionoperator for the mass of data.

If steep dip is encountered on the data from the shorter group, then itis impractical to use the conventional Pie-slice or Fan-filtertechniques (which combine data from a series of CRPs) with gates setonly for near horizontal reflections. If steep dip is encountered in onedirection only, then it would be possible to use Pie-slice orFan-filters on data from the longer group; however, the series of dipgates would be centered about the dip encountered on data from theshorter group of detector arrays. This reduces the time required tosearch for both steep dips and slight dips in both directions throughoutthe large mass of data.

A further alternative includes recording both the shorter group and thelonger group of detector arrays using the same low-cut (high pass)filter but with a higher frequency for the high-cut filter on theshorter group than on the longer group of detector arrays. For example,filter 202 in FIG. may have a pass band of 5 to 300 Hz. If this is done,the monitor for the shorter group uses an additional low-cut (eitheranalog or digital) filter to attenuate low-frequency noise for themonitor record. However, the data from several or all arrays in theshort group may have had NMO applied and then be combined to eliminatethe low-frequency noise rather than using filtering. The total length ofthe shorter group of detector arrays is comparable to thecenter-to-center distance between consecutive arrays in the longer groupof detector arrays. The combination of these two groups will attenuatethe lowfrequency noise. The high-frequency signals will not beattenuated (except when very steep dip is encountered) since applicationof NMO brought the signals into coincidence before combination. Hencethis combination of all of the arrays in the shorter group provides awider frequency response than an array in the longer group of detectorarrays. This broad pass band of frequencies can be used to determine theoptimum filter which should be used for the deep reflections as well asthe shallow reflections and whether a time varying filter should beapplied to the subsequent processing of the data from the longer groupof detector arrays. Considerable processing time can be saved byomitting processing programs that are redundant for the particular databeing processed. This combined data from the shorter group also isindicative of whether higher frequencies would be useful on the datafrom the longer group of detector arrays in a given area. If so, thefrequency response and sampling of the data from the longer group ischanged where indicated. The preceding is one alternative fonn of theinvention.

The shorter group provides better identification of bubble collapse andoscillation of the seismic source. Such bubble collapse and oscillationis hard to identify on low-frequency records where the time between thesource and the collapse is short. This collapse is readily observed onthe autocorrelation of the data from the shorter group. Theidentification and determination of the characteristics of the bubbleoscillation are useful not only in subsequent processing of the datafrom the longer group but also in planning a procedure to reduce thisoscillation at the seismic source or at least shifting the frequencyoutside the seismic frequency pass band of the recording system.

Those skilled in the art will recognize other modifications of theapparatus and method. While preferred embodiments of the invention havebeen shown and described, it will be apparent that changes may be madewithout departing from the principles and spirit of the invention, thescope of which is defined by the appended claims. Accordingly, theforegoing embodiments are to be considered illustrative only, ratherthan restrictive of the invention, and those modifications which comewithin the meaning and range of equivalency of the claims are to beincluded.

Having thus described the invention, what is claimed as new and desiredto be secured by Letters Patent is:

1. A method for obtaining seismic reflection signals, comprising thesteps of:

a. locating first and second group of seismic detectors along a traverseto respectively provide high and low multiplicity of coverage b. saidfirst group including N, detectors with a detector interval of D, andsaid second group including N detectors with a detector interval of D,,

c. detennining the multiplicity of coverage M, of said first group and Mof said second group in accordance with the formula,

d. simultaneously recording seismic signals from said first and secondgroup of detectors successively from seismic sources at differentlocations.

2. A method as in claim 1 wherein said first group of detectors islocated more remotely from a seismic source than said second group ofdetectors.

3. A method as in claim 1 wherein the seismic reflection signalsreceived from said second group of detectors are passed through a higherfrequency band-pass filter than the seismic reflection signals from saidfirst group of detectors prior to recording said seismic signals.

4. A method as in claim 1 wherein the seismic signals from said secondgroup of detectors are sampled more frequently than the seismic signalsreceived from said first group of detectors 5. A method as in claim 1wherein said first and second groups of detectors are locatedsuccessively along a single marine cable.

6. A method as in claim 1 wherein said first and second groups ofdetectors are respectively located on at least two cables.

7. A method as in claim 1 wherein the multiplicity of coverage of saidfirst group of detectors is higher than the multiplicity of coverage ofsaid second group of detectors.

8. Apparatus for obtaining seismic reflection signals, comprising:

first and second groups of seismic detectors located successively alonga traverse to respectively provide high and low multiplicity ofcoverage,

said first group including N, detectors with a detector interval of D,and said second group including N detectors with a detector interval ofD,,

said first group and said second group of detectors respectivelyproviding M, and M multiplicity of coverage in accordance with theformula,

and means for individually recording the seismic signals from said firstand second groups of detectors.

9. Apparatus as in claim 8 wherein said first group of detectors arelocated more remotely from a seismic source than said second group ofdetectors.

10. Apparatus as in claim 8 further comprising first and secondfrequency band-pass filters and said second frequency band-pass filterhas a higher frequency band-pass than said first frequency band-passfilter, said first and second filters respectively filtering the seismicsignals from said first and second groups of detectors.

11. Apparatus as in claim 8 further comprising means for sampling theseismic signals received by said first and second groups of seismicdetectors and wherein the sampling rate is marine cable.

13. Apparatus as in claim 8 wherein said first and second groups ofdetectors are located along at least two marine cables.

82233 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.2513.071 Dated October 12, 1971 Inventor(s) Roy G Q y It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

' Column 1, line 5A, "or" should read. --if--; Column 1, line 67,low-frequency should read --changes--; Column 2, line 39, "12" shouldread --la--; Column 2, line 61, "2 should read -3 L-; Column 3, line 60,"reflections" should read reflection--5 Column line 35, "the detectorlocations'? should read --the spacing of the detector 1ocations--;Column L, line #6, "sued" should be --used--; Column 1- line 67, 'st"should be omitted; Column 7, line 5 T, group should read roup.--; Column7, line 57, "6 Di/E" should read --6xD /2--;

Column 10, line 61, N =D /M shou d read -N1XD1/M1-.

Signed and sealed this 15th day of April 1972.

(SEAL) Attest: a

EDWARD MJ LETCIILJQJli. HOLBL'MI GOTTSCHALK Attesting OfficerCommissioner of Patents

1. A method for obtaining seismic reflection signals, comprising the steps of: a. locating first and second group of seismic detectors along a traverse to respectively provide high and low multiplicity of coverage b. said first group including N1 detectors with a detector interval of D1 and said second group including N2 detectors with a detector interval of D2, c. determining the multiplicity of coverage M1 of said first group and M2 of said second group in accordance with the formula, N1 X D1/M1 N2 X D2/M2 and d. simultaneously recording seismic signals from said first and second group of detectors successively from seismic sources at different locations.
 2. A method as in claim 1 wherein said first group of detectors is located more remotely from a seismic source than said second group of detectors.
 3. A method as in claim 1 wherein the seismic reflection signals received from said second group of detectors are passed through a higher frequency band-pass filter than the seismic reflection signals from said first group of detectors prior to recording said seismic signals.
 4. A method as in claim 1 wherein the seismic signals from said second group of detectors are sampled more frequently than the seismic signals received from said first group of detectors
 5. A method as in claim 1 wherein said first and second groups of detectors are located successively along a single marine cable.
 6. A method as in claim 1 wherein said first and second groups of detectors are respectively located on at least two cables.
 7. A method as in claim 1 wherein the multiplicity of coverage of said first group of detectors is higher than tHe multiplicity of coverage of said second group of detectors.
 8. Apparatus for obtaining seismic reflection signals, comprising: first and second groups of seismic detectors located successively along a traverse to respectively provide high and low multiplicity of coverage, said first group including N1 detectors with a detector interval of D1 and said second group including N2 detectors with a detector interval of D2, said first group and said second group of detectors respectively providing M1 and M2 multiplicity of coverage in accordance with the formula, N1 D1/M1 N2 X D2/M2 and means for individually recording the seismic signals from said first and second groups of detectors.
 9. Apparatus as in claim 8 wherein said first group of detectors are located more remotely from a seismic source than said second group of detectors.
 10. Apparatus as in claim 8 further comprising first and second frequency band-pass filters and said second frequency band-pass filter has a higher frequency band-pass than said first frequency band-pass filter, said first and second filters respectively filtering the seismic signals from said first and second groups of detectors.
 11. Apparatus as in claim 8 further comprising means for sampling the seismic signals received by said first and second groups of seismic detectors and wherein the sampling rate is higher for the seismic signals from said second group of detectors than from said first group of detectors.
 12. Apparatus as in claim 8 wherein said first and second groups of detectors are successively located along a single marine cable.
 13. Apparatus as in claim 8 wherein said first and second groups of detectors are located along at least two marine cables. 